The present invention relates to an apparatus wherein when an image formed on an image carrier is developed by a plurality of developing units, a developing sleeve provided in each developing unit is controlled not to start rotating promptly in the course of development so that a photoreceptor drum for image forming may not be adversely affected.
An image forming apparatus has hitherto been provided at least with a charging unit, an exposure unit employing an ordinary light source or a laser beam, and a developing unit only for black images or a plurality of developing units, in the case of color copying, all arranged in succession around a photoreceptor drum which is an image carrier. When forming an image on the photoreceptor drum, the charging unit charges the entire surface of the photoreceptor drum with electricity first and the exposure unit exposes the surface of the photoreceptor drum depending on input signals from a document or from a computer, thus, there is formed a latent image corresponding to the signals coming from the document or the computer. The latent image is then developed by the developing unit. In the case of the color copies in particular, there are provided a plurality of developing units each of which contains different color toner, and color developing is carried out in a way wherein only the developing unit that takes part in developing among other developing units is brought into contact with the surface of the photoreceptor drum for developing operation. The developing unit is generally provided with a developing rotary sleeve which is rotated by a driving means at high speed (for example, 300-350 rpm depending on colors) in the course of developing so that color toner may be conveyed to the latent image to cover all kinds of images for excellent developing.
In the situation that the driving means is started to rotate the developing rotary sleeve for developing a latent image formed on the photoreceptor drum as described above, the diameter of the photoreceptor drum can not be so large due to the general restriction in terms of total dimensions and a cost of an apparatus. Even in the case of a small document size used normally, therefore, the exposure unit is still working for exposure even when a latent image formed on the photoreceptor drum arrives at the developing unit and the developing rotary sleeve starts rotating. In a method of exposure, a portion charged with electricity on the surface of the photoreceptor drum is then subjected to exposure in succession generally. When a laser beam is used for exposure, in particular, scanning for a small width from side to side which is synchronized with a rotation of the photoreceptor drum is conducted for exposure. When a developing rotary sleeve of a developing unit is started under the condition mentioned above, the higher the speed of rotation of the sleeve is, the greater the shock in the start is, and the surface of the photoreceptor drum is naturally affected by the shock. The reason for this lies in that the thickness of a developer layer located on a developing rotary sleeve to be conveyed in the recent developing unit is thin, the distance between the developing rotary sleeve and the surface of a photoreceptor drum (hereinafter referred to as DSD) is extremely small, and a DSD width which is as small as 0.5 mm is required to be maintained accurately. As a means for forming the DSD width, there are provided rollers at both sides of the developing rotary sleeve so that the rollers may be in contact with the surface of the photoreceptor drum to maintain the accurate DSD width. Since the rollers are provided on a coaxial basis with the developing rotary sleeve, the shock in the start is transmitted to the photoreceptor drum through the rollers and accurate rotation of the photoreceptor drum is affected adversely temporarily. Therefore, a jittering problem is caused, at the start of operation, on the latent image forming surface subjected to exposure, resulting in a defective developed image. When forming a color image by superimposing different plural colors, in particular, the jittering problem is caused for each developing unit, making it impossible to obtain color images, which is a disadvantage.